Synthesis, characterization, and X-ray crystal structures of cobalt(II) and cobalt(III) complexes of four topologically constrained tetraazamacrocycles
作者:Timothy J Hubin、Nathaniel W Alcock、Howard J Clase、Lawrence L Seib、Daryle H Busch
DOI:10.1016/s0020-1693(02)01080-0
日期:2002.9
The high spin Co2+ complexes of 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2), 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3), and racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4) have been synthesized and characterized by X-ray crystallography. The Co(III) complexes of 1-3 were also prepared by the chemical oxidation of the Co(II) complexes. The X-ray crystal structures of Co(1)Cl-2, Co(3)Cl-2, and [Co(3)Cl-2]PF6 demonstrate that the ligands enforce a distorted octahedral geometry on Co(II) and Co(III) with two cis sites occupied by chloro ligands. In contrast, the Co(II) complex of 4 is five-coordinate with trigonal bipyramidal geometry. The methyl groups substituted on the carbon atoms of ligand 4 define a shallow cavity, allowing only one chloride ligand to bind to the chelated metal ion. This difference in coordination geometry causes Co(4)Cl+ to be much more difficult to oxidize (E-1/2 = 1.176 V vs. SHE) than the octahedral Co(II) complexes of 1-3 (E-1/2 from -0.157 to 0.173 V vs. SHE). The Co(III) complexes of 1-3 have three absorbances in their electronic spectra, while typical cis-Co(III)N4X2 complexes have only two. This additional splitting of energy states is attributed to the increased distortion from octahedral resulting from the short ethylene cross-bridge on the macrobicyclic ligands. (C) 2002 Elsevier Science B.V. All rights reserved.